Where did this idea of "eat the flesh of the Son of man" come from? Why would Jesus use such an expression anyway? Is there a way to look at this expression and make sense of it? Yes, there is!
The first place this idea was used is in Exodus chapter 12. The setting was the first passover when each household of Israel took a lamb, kept it for four days, sacrificed it to put the blood upon the doorposts and lintel, roasted the lamb whole and the entire household ate that sacrificial lamb. Here we recall John the Baptist's words in John 1:29, "The next day John sees Jesus coming to him, and says, Behold the Lamb of God, who takes away the sin of the world." That first passover lamb pointed toward the ultimate passover Lamb. The first lamb was to be completely eaten (with inedible parts burned afterwards) so that the eaters took into themselves the lamb. "Eat not of it raw, nor sodden at all with water, but roast with fire; his head with his legs, and with the purtenance thereof. And ye shall let nothing of it remain until the morning; and that which remaineth of it until the morning ye shall burn with fire." (Exo 12:9-10)
It was to be a physically performed ceremony, repeated throughout their generations, to engage their thinking and point to Jesus as that promised Savior. The priests in the Tabernacle (and later, the Temple) services also ate of the sacrifices pointing to the same idea of "eating" or taking into themselves the purpose for and benefit of that sacrifice. Those priests even laid their hands upon the heads of those often repeated sacrifices implying that the animal(s) stood in their place. What occurred to the animal was what should have happened to themselves—death for their sin(s). To bring together the first passover with His imminent sacrifice as the "Lamb of God," Jesus used the expression of "eating" which should have been very familiar to all of them who listened to God's words with their heart and head.
Jesus spoke of part of His purpose for coming to earth when he said, "For I have given unto them the words which thou gavest me; and they have received them, and have known surely that I came out from thee, and they have believed that thou didst send me." (John 17:8) To fulfill this Matthew tells us, "And Jesus went about all the cities and villages, teaching in their synagogues, and preaching the gospel of the kingdom, and healing every sickness and every disease among the people." (Matt 9:35) Luke agrees saying, "And he went through the cities and villages, teaching, and journeying toward Jerusalem." (Luke 13:22)
This showed that the "words" or "teachings" of Jesus was the substance of his statements (Luke 9:6; John 14:23; 15:7), "he that eateth me, even he shall live by me" (John 6:57b), "If any man thirst, let him come unto me, and drink." (John 7:37b) and "It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life." (John 6:63). This was validated by the chief priests in Luke 23:5, "And they were the more fierce, saying, He stirreth up the people, teaching throughout all Jewry, beginning from Galilee to this place." The scribes, Pharisees, Sadducees, chief priests and others were not upset about the physical body of Jesus but about the import and implications of his words and deeds.
Now, of course, eating the flesh brought the food value into the person and the indigestible parts were excreted. The nutrition from eating was transferrd into the blood stream and transported throughout the body for cellular maintenance and repair. That physical nutrition was wholly dispersed and became incorporated into or integral with the body. However, the "Lamb of God" was not to be physically eaten but spiritually eaten.
We, according to Jesus' words, "Verily, verily, I say unto you, He that believeth on me hath everlasting life. I am that bread of life." (John 6:47-48) are to eat his flesh, i.e. to take spiritually into ourselves the benefit of His teachings and of His perfect human sacrifice on our behalf for our sins. Both Jesus' words and His sacrifice for us are to become wholly absorbed so that it becomes an integral part of who we are (new creatures), what we know (truth), what we stand for (righteousness), what we do (love one another), and how we live (sacrificially) that we become one with Him (His Bride).
This is why Jesus said, "It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life." (John 6:63) Apostle Paul expressed exactly the same thing when he said, "And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power. ... Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual." (1 Cor 2:4, 13) The message of God is to reach into our hearts and change our hearts to recognize and discern "truth." "For the law was given by Moses, but grace and truth came by Jesus Christ." (John 1:17); "I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth." (1 John 2:21)
Apostle John said, "And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth." (John 1:14) Jesus explained, "But the hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship him." (John 4:23) And, "When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you? What and if ye shall see the Son of man ascend up where he was before? It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. But there are some of you that believe not." (John 6:61-64a)
"Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it?" (John 6:60) Yes it is! But only if we think of Jesus' flesh as the sum and substance of His being can we come to this conclusion. This thinking is carnal and the result is as the scripture says, "For to be carnally minded is death; but to be spiritually minded is life and peace. Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be." (Rom 8:6-7)
"From that time many of his disciples went back, and walked no more with him." (John 6:66) If we want to take just some of Jesus' words into our hearts and refuse the rest or take some of His sacrifice for our sin and add to it our efforts we are doing exactly the same thing. By this method we can reject Him and He in turn will also reject us.
As for me and my household, "For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith." (Rom 1:16-17)